Where to start depends on your level of knowledge, and you didn’t give any information on that. Is this your first arranger, how competent is your musical ability etc.
If this is your first arranger start by learning to select a style, how to drive it, select a RH voice and start playing and experimenting. Then learn how to set/change tempo, different fingering modes and how to use the music finder or whatever it is called on Gemos and keep playing.
Registrations, altering styles, recording, etc are WAY down the line. Don’t get bogged down too soon on the deep operating system. Just get making music at your level. When you need to know something read the manual first, but if this doesn’t make it clear - and it often will not - ask a clear question here.
Mike